Cooling water system

ABSTRACT

A system for supplying cooling water to a process on board a floating vessel for the production of hydrocarbons, wherein the vessel ( 1 ) is anchored by means of a bottom-anchored turning unit ( 20 ) mounted in a receiving space ( 7 ) in the hull ( 34 ) of the vessel and allowing turning of the vessel ( 1 ) about the turning unit, and wherein the turning unit ( 20 ) supports a swivel unit ( 24 ) for the transfer of hydrocarbons from production risers ( 28 ) extending between the seabed and the turning unit ( 20 ), the system comprising a conduit means ( 30 ) depending from the vessel ( 1 ) to a depth for taking in cooled sea water, and a pump means ( 44 ) for pumping of the sea water from the conduit to a place of use for the process. The turning unit ( 20 ) is designed as a seawater swivel, the unit being provided with one or more passages ( 29 ) for receiving upper end portions of respective seawater risers ( 30 ) constituting the conduit means, and with a means for transferring sea water from the upper end portions of the risers ( 30 ) to an annulus ( 31 ) arranged at the boundary surface between mutually movable parts ( 21, 22 ) of the turning unit ( 20 ) or between the tuning unit ( 20 ) and the vessel hull ( 34 ), and communicating with one or more passages ( 41 ) arranged in the vessel hull and leading to said place of use, a seawater sealing means ( 37, 39 ) being arranged on each side of the annulus ( 31 ).

[0001] The invention relates to a system for supplying cooling water toa process on board a floating vessel for the production of hydrocarbons,wherein the vessel is anchored by means of a bottom-anchored tuning unitmounted in a receiving space in the hull of the vessel and allowingturning of the vessel about the turning unit, and wherein the turningunit supports a swivel unit for the transfer of hydrocarbons fromproduction risers extending between the seabed and the turning unit, thesystem comprising a conduit means depending from the vessel to a depthfor taking in cooled sea water, and a pump means for pumping of the seawater from the conduit to a place of use for the process.

[0002] Offshore extraction and production of hydrocarbons in many casesis carried out on board so-called FPSO vessels, i.e. vessels constructedand built for production, storage and offloading of hydrocarbons(FPSO=Floating Production, Storage and Offloading).

[0003] Such vessels are typically anchored by means of a pluralityanchor lines fixed to anchors on the seabed and to a turning unitmounted in a receiving space in the hull of the vessel, and allowing thevessel to turn freely about the turning unit, under the influence ofwind, waves and water currents. The turning unit may be a submerged buoyof the two-part type comprising a bottom-anchored central member and anouter buoyancy member which is rotatably mounted on the central memberand is releasably fastened in the receiving space in the vessel hull. Asan alternative, the turning unit may consist of a bottom-anchoredturning body (turret) which is rotatably mounted in the receiving spaceby suitable bearing means, or is rotatably suspended from the deck or inthe bow of the vessel.

[0004] As the turning unit allows the vessel to turn freely about theanchoring point, its central buoy member or turning body, which isstationary in relation to the seabed, supports a swivel unit for thetransfer of process fluids etc. between the relevant risers and a pipesystem on the vessel. The risers transfer oil, gas and water between thevessel and the seabed, and there is further arranged a so-calledumbilical providing paths for chemicals, electric and fibre-opticsignals, and electric and hydraulic power.

[0005] A process plant on board a vessel of the above-mentioned typerequires supply of large quantities of cooling water. A typical FPSOvessel for oil production may use about 5000 m³/h, and an LNG planttypically may require about 30000 m³/h. Most FPSO vessels today utilizea cooling water intake structure which, by means of pumps, pulls up seawater to a seawater intake via freely hanging, flexible hoses orconduits extending down to a depth of maximum 40 m. As mentioned above,the vessel is anchored by means of a plurality of anchor lines fastenedto the turning unit. This implies that the length of the seawater intakepipes is limited to avoid interfering collisions with the anchor lines.From the water intake the sea water is pumped further to cooling deviceson the vessel. Because of the limited length of the cooling water intakepipes, the temperature of the intake water is almost the same as thesurface temperature.

[0006] The efficiency of a process comprising cooling increases withincreasing temperature of the cooling water. The result is a lowerenergy consumption and a more efficient, and therewith less expensiveequipment. As known, the temperature of the sea water decreases with thewater depth, so that it is generally advantageous to have the seawaterintake as deeply as possible.

[0007] The object of the invention is to provide a system for the supplyof cooling water for the current purpose wherein the system enables avery cost-efficient and operationally safe construction for coolingwater supply, and simultaneously enables the supply of sea water withthe lowest possible temperature to the cooling systems of the vessel.

[0008] The above-mentioned object is achieved with a system of theintroductorily stated type which, according to the invention, ischaracterized in that the turning unit is designed as a seawater swivel,the unit being provided with one or more passages for receiving upperend portions of respective seawater risers constituting the conduitmeans, and with a means for transferring sea water from the upper endportions of the risers to an annulus arranged at the boundary surfacebetween mutually movable parts of the turning unit or between theturning unit and the vessel hull, and communicating with one or morepassages arranged in the vessel hull and leading to said place of use, aseawater sealing means being arranged on each side of the annulus.

[0009] In the system according to the invention, the cooling water pipesare located within the anchoring system and are geostationary inrelation to the seabed, and thus they will not interfere with theanchoring system and the production risers when the vessel turns underthe influence of wind and weather. The cooling water pipes therewith maybe extended all the way down to the seabed without interfering with theanchoring system. The cooling water is not passed through the processswivel, but is passed directly through the turning unit and into thevessel by the use of simple dynamic and static seals.

[0010] The system is particularly valuable in places where the air andseawater surface temperatures are high. The lower cooling watertemperature implies a number of economic and environmental advantages.As to economic advantages, there may be mentioned:

[0011] Stable annual production quantities

[0012] Constant cooling water temperature facilitates optimum processoperation

[0013] Increased production in relation to power consumption

[0014] Lower maintenance costs because of lesser fouling and corrosiontendency of the cold sea water

[0015] Lower condensation temperature for the steam turbine increasesits output

[0016] Lower design pressure for the fractionating and cooling part ofthe production plants

[0017] Reduced heat transfer surface area because of less fouling andlower AT

[0018] A more compact process plant design which is better suited forFPSO vessels

[0019] Lower cost for the process plant

[0020] As to environmental advantages, there may be mentioned:

[0021] Lesser CO₂ spill in relation to production quantity

[0022] No chlorinating necessary

[0023] Practically no thermal contamination

[0024] The invention will be further described below in connection witha number of exemplary embodiments with reference to the drawings,wherein

[0025]FIG. 1 shows a side view of a vessel which is anchored to a seabedand is provided with a cooling water supply system according to theinvention;

[0026]FIG. 2 shows a schematic sectional view of a first embodiment of asystem according to the invention;

[0027]FIG. 3 shows a schematic sectional view, as viewed from above, ofa part of a vessel hull with elements forming part of a system accordingto the invention;

[0028]FIG. 4 shows a schematic side view of the arrangement of FIG. 3;

[0029]FIG. 5 shows a sectioned side view of a wing tank having a suctionextension well;

[0030]FIG. 6 shows a schematic sectional view of a second embodiment ofa system according to the invention;

[0031]FIG. 7 shows a schematic sectional view of a third embodiment of asystem according to the invention;

[0032]FIG. 8 shows a schematic side view, partly in section, of a fourthembodiment of a system according to the invention;

[0033]FIG. 9 shows a side view of an embodiment essentiallycorresponding to the embodiment according to FIG. 2;

[0034]FIG. 10 shows the detail A in FIG. 9 on an enlarged scale;

[0035]FIG. 11 shows a sectional view essentially along the line XI-XI inFIG. 10;

[0036]FIG. 12 shows a corresponding sectional view to that of FIG. 11,but of an alternative embodiment;

[0037]FIG. 13 shows a sectional view of a fifth embodiment of a systemaccording to the invention;

[0038]FIG. 14 shows a sectional view essentially along the line XIV-XIVin FIG. 13; and

[0039]FIG. 15 shows a corresponding sectional view to that of FIG. 14,but of an alternative embodiment.

[0040] In the drawings, corresponding parts and elements in thedifferent drawing figures are designated by the same reference numerals.

[0041] In FIG. 1 there is shown an FPSO vessel 1 floating on a watersurface 2 and being anchored to a seabed 3 by means of a plurality ofanchor lines 4. The anchor lines at their lower ends are connected torespective anchors 5, and at their upper ends they are connected to aturning unit 6 mounted in a submerged receiving space 7 at the bottom ofthe vessel. As mentioned above, the anchor lines are connected to acentral buoy member or a turning body (turret) allowing the vessel toturn freely about the anchoring point. As also mentioned above, thegeostationary turning body or buoy member supports a swivel unit (notshown in FIG. 1) for the transfer of, inter aria, hydrocarbons from oneor more production risers 8 extending between the seabed 3 and theturning unit 6.

[0042] The system of the vessel 1 for the supply of cooling water toproduction processes on the vessel includes one or more seawater risers9 which are shown to extend between the turning unit 6 and the seabed 3,and which are connected at their lower end to an anchoring means on theseabed, for instance a seawater lifting pump 10. In the illustratedembodiment, both the production risers 8 and the seawater risers 9 areshown to comprise an upper flexible part which, at its lower end, isconnected to a buoyancy unit 11 for support of the risers, and a lowerpart extending between the buoyancy unit 11 and the seabed 3. A seawaterlifting pump 12 is also shown to be arranged on the buoyancy unit 11.The buoyancy unit 11 is moored to the seabed by means of mooring lines13 connected at their lower ends to respective anchors 14.

[0043] The seawater risers 9 generally may consist of one large orseveral smaller risers extending down to the seabed or to a chosen depthat which the seawater temperature is sufficiently low. As also appearsfrom FIG. 1, the water pipes 9 between the buoyancy unit 11 and theseabed 3 may have the same course as the production risers 8, or theymay extend generally vertically from the buoyancy unit to the seabed. Inboth cases they will be kept in position at the seabed by means of ananchoring means.

[0044] A first embodiment of the system according to the invention isshown in FIG. 2. The figure shows a cross-section of a vessel 1 providedat the bottom of the vessel with a receiving space 7 for the receipt ofa turning unit which, in the illustrated case, is constituted by atwo-part submerged buoy 20 comprising a bottom-anchored central member21 and an outer buoyancy member 22 which is rotatably mounted on thecentral member. The central member is anchored by means of a suitablenumber of anchor lines 23. The central member supports a swivel unit 24which, in a usual manner, may comprise a process swivel 25, a hydraulicutility swivel 26 and an electric power and control signal swivel 27.Further, the central member supports a number of process or productionrisers 28 extending between the process swivel 25 and the seabed (notshown).

[0045] In accordance with the invention, the turning unit or buoy 20 isdesigned as a seawater swivel, i.e. a swivel for transferring sea water.For this purpose the central member 21 of the buoy is provided with anumber of passages 29 receiving the upper end portions or respectiveseawater risers 30, and with a means for the transfer of sea water fromthe risers to an annulus 31 arranged at the boundary surface between thecentral member 21 of the buoy and its outer buoyancy member 22. In theouter member of the buoy there is arranged a number of radial passages32 communicating with an additional annulus 33 arranged at the boundarysurface between the outer member 22 and the vessel hull 34.

[0046] As appears, the seawater risers 30 are closed at their upper endby means of a lid 35, and they are provided with water outlets in theform of a plurality of holes 36 communicating with the annulus 31between the inner and outer members 21, 22 of the buoy. Outside of theoutlet holes 36, the risers 30 suitably may be surrounded by respectiveannuluses communicating with the annulus 31 between the buoy membersthrough a number radial passages in the inner buoy member 21.

[0047] On each side of the annuluses 31 and 33 there are arrangedrespective sealing means, more specifically inner sealing means 37 and38, respectively, preventing leakage of sea water into the space abovethe buoy 20, and outer sealing means 39 and 40, respectively, preventingleakage of warmer surface sea water into the passages for cold waterfrom the risers 30. As will be understood, it is here the question ofdynamic sealing means 37, 39 between the mutually movable buoy members,and static sealing means 38, 40 between the outer buoy member and thevessel hull.

[0048] In the vessel hull there are arranged a number of passages 41extending between the annulus 31 and a water intake in the vessel. Inthe illustrated embodiment, this water intake is constituted by a pairof wing tanks 42 arranged on respective sides of the vessel 1. Thepassages 41 lead into the wing tanks 42 via a respective valve 43, andare associated with a pump means 44 connected to an appurtenant conduit45 for the supply of water in the wing tank to the relevant place of usein the production process on the vessel.

[0049] The annulus 33 between the outer buoy member 22 and the vesselhull 34 possibly might be omitted under the presupposition that the buoy20 were provided with suitable guiding means ensuring that the buoy isintroduced and secured in the receiving space with the passages 32aligned with respective ones of the passages 41 in the vessel hull.

[0050] As mentioned in the introduction, a process plant on an FPSOvessel requires large quantities of cooling water, typically 5000 to30000 m³/h. The taking-in of such large water quantities through aswivel will require a flow area corresponding to a pipe having adiameter from ca. 500 mm up to ca. 2000 mm. Swivels for the transfer ofwell flows normally have a flow area corresponding to pipes having aninner diameter from 10 mm up to 400 mm. Swivels for well flows have toseal completely for well flows having a pressure of up to 300-400 bar,because any leakage of process fluid may be critical. The design of suchswivels and associated sealing systems requires special materials,strict tolerances and expensive sealing systems. A possible smallleakage in a swivel transferring sea water is unproblematic, and aswivel for sea water may be designed for a low pressure (typically 1-5bar), with simple components, cheaper materials and simpler sealingsolutions.

[0051] The central buoy member or turret will be subjected to high loadsfrom the anchoring system. The turret therefore has a limited capabilityof accepting pressure in a seawater passage. However, installing thepumps in a sea water intake in a wing tank as shown in FIG. 1, willlower the pressure inside the turret. The turret therefore will not beIs unduly stressed in its application as a seawater swivel. Even if thepumps in some cases will have to be lowered down into the seawaterrisers, as described below, the pressure of the water can be kept verylow. The extra stress on the turret can also be kept low.

[0052]FIGS. 3 and 4 show a schematic plan view and a side view,respectively, of a part of the elements shown in FIG. 2. As appears fromFIG. 3, the passages 41 consist of six pipes of which three pipesdebouch into each of the wing tanks 42 via a respective valve 43. Ineach of the wing tanks there are arranged four seawater lifting pumps44. At the top of the conduits 45, extending between the pumps and thedeck of the vessel, there is arranged a unit 46 for electric powersupply to the associated pump.

[0053] In each of the wing tanks 42 there is also arranged an emergencywater inlet means, more specifically three emergency inlets 47communicating with the surrounding sea via appurtenant valves 48. Thevalves 43 and 48 are shown to be coupled to a valve handle 49 and 50,respectively, at the deck of the vessel 1, for operation of the valves,either manually or by remote operation. The emergency inlets are used ifthe water passages or the inlet valves 43 should be damaged, so that thecooling water flow is limited. Water flowing into the wing tanks in caseof opening of the emergency inlets, will be water from the vicinity ofthe surface, and thus have a higher temperature. However, the processthen may still be supplied with cooling water even if it has a higherinlet temperature.

[0054] When the inlet valves 43 in the wing tanks are opened, there willbe a free passage for the water from the inlet at the lower end of theseawater risers to the wing tanks. When the pumps 44 start working, thewater level in the wing tanks start dropping, as suggested in FIG. 2.The difference in static height between the inside and outside of theseawater intake or wing tank pushes the water up through the risers 30,through the central buoy member (turret) and through the passages andinto the wing tanks. The water level within the wing tanks will dropuntil there is a balance between the friction losses in the pipes andpassages and the pressure created by the difference in static height ofthe water. To ensure that the difference in level will not be too high,the inside diameter of the seawater risers is so large that anacceptable friction loss is generated, estimated to 5-10 m of watercolumn.

[0055] If the water level inside the water intake or wing tank is toolow, the pumps 44 may cavitate and be damaged. To ensure that the pumpshave a sufficient pressure at the inlet of the impeller, a hole can bemade in the bottom of the wing tank, and the pump can be placed in asuction extension well in the form of a container installed below thetank bottom. Such an embodiment is shown in FIG. 5 wherein a container55 is installed in an opening in the bottom of the tank 42 and receivesa pump head 44. The container and the pump head may be installed fromthe deck and may be lifted out as a unit if desired. A seal (not shown)is provided between the container and the vessel hull 34, to prevent“warm” surface water from leaking into the wing tank.

[0056] A second embodiment of the system according to the invention isshown in FIG. 6. The embodiment to a large extent corresponds to theembodiment of FIG. 2, but the seawater pumps here are not arranged in awater intake in the vessel. Instead a pump 56 is arranged in each of theseawater risers 30 at a location below the buoy 20. Electric power tothe pumps is supplied as shown via the swivel unit 24 and coupling heads57 at the top of the risers 30. In this embodiment, instead of thepassages 41 in the vessel hull shown in FIG. 2, there are arranged anumber of passages 58 which are connected to respective conduits 59extending upwards in the space 60 above the buoy and supplying coolingwater to the relevant place of use in the production process on thevessel.

[0057] A third embodiment of the system according to the invention isshown in FIG. 7. Also this embodiment to a large extent corresponds tothe embodiment of FIG. 2, except that the seawater pumps are notarranged in a water intake in the vessel. Instead, the relevant pumps 61are arranged in the space 60 above the buoy 20. The pumps are driven byappurtenant motors (M) 62 arranged in a pump room 63 wherein also thepumps may be arranged. The pumps 61 are connected to passages orconduits 64 communicating with the passages 32 in the outer buoy member,possibly via an annulus (not shown), as in the embodiment according toFIG. 2.

[0058]FIG. 8 shows a schematic, partly sectioned side view of a fourthembodiment of a system according to the invention. In this case theturning unit is constituted by a bottom-anchored turning body (turret)70 mounted in a receiving space 71 arranged in a vessel 1 at a levelabove the water surface 72, more specifically in a hull part 73extending forwards from the bow of the vessel 1. The turning body isrotatably mounted in relation to the receiving space, so that the vesselcan turn freely about the turning body. The anchor lines forbottom-anchoring of the turning body are omitted in FIG. 8.

[0059] The turning body is provided with a number of vertical passagesfor receiving the upper end portions of risers 30, these portions, in amanner similar to the embodiment according to FIG. 2, being providedwith a number of outlet holes 74 for sea water. The outlet holescommunicate with radial passages 75 leading to an annulus 76 between theturning body and the hull part 73. A pipe connection 77 is arrangedbetween the annulus 76 and the relevant place of use on the vessel.Dynamic seals 78 and 79 are arranged on each side of the annulus 76.

[0060] In this embodiment in which the turning body is arranged abovethe water surface, the water will not flow in the system withoutartificial lift. The seawater pumps therefore must be installed withinthe seawater risers 30. A pump 80 is shown to be installed in each ofthe risers 30 at a sufficient depth H below the water surface to producea sufficient static pressure to ensure that the pump has suitablesuction conditions. A typical distance is 10-40 m below the watersurface. As the turret and pumps 80 are stationary in relation to theseabed, the power supply to the pumps must take place via the swivelunit 24 and respective junction boxes 81. In addition to the pumps 80,also a booster pump 82 is shown to be arranged in the pipe connection77.

[0061]FIG. 9 shows a sectional view of an embodiment which in allessentials corresponds to the embodiment according to FIG. 2, butwherein the Figure shows some additional details and constructionalmodifications, especially in connection with the buoy 20. For adescription of the embodiment reference is made to the description ofFIG. 2. In addition it may be remarked that the Figure also shows alocking mechanism 85 for releasable attachment of the buoy 20 in thereceiving space in the vessel.

[0062]FIG. 10 shows a cutout A in FIG. 9 on an enlarged scale, and showsconstruction-details in connection with the annuluses 31 and 33 and thesealing means 37-40.

[0063]FIG. 11 shows a horizontal section along the line XI-XI in FIG. 9and shows a possible arrangement of production risers 28 and seawaterrisers 30 in the central buoy member 21. As shown, there are arrangedseven production risers 28 and six seawater risers 30 which aredistributed along respective concentric circles. Each of the seawaterrisers 30 outside of the outlet holes 36 is partly surrounded by apassage 86 communicating with the annulus 31. The annulus 31 in turncommunicates with the annulus 33 via three passages 32.

[0064]FIG. 12 shows a sectional view corresponding to that of FIG. 11,but of an alternative embodiment with respect to the connection betweenthe riser outlets 36 and the passages 32. This embodiment is withoutindividual passages (or annuluses) in connection with each of theseawater risers 30. Instead, the annulus 31 is radially extended to alarger annulus 87, and placed such that the outlet openings 36 of therisers debouch directly into this annulus.

[0065]FIG. 13 shows a sectional view of a fifth embodiment of the systemaccording to the invention.

[0066] In a manner corresponding to FIG. 8, the turning unit here isconstituted by a turning body 70 which is rotatably mounted in areceiving space in the vessel 1, but the receiving space here is in theform of a submerged well 90 arranged in the bottom of the vessel. Theturning body is supported by a bearing means consisting of an axialbearing 91 and a radial bearing 92. The turning body is anchored to theseabed by means of a number of anchor lines 93 (only one is shown)introduced into the turning body via respective guide tubes 94.

[0067] In a manner corresponding to FIG. 8, the seawater risers 30 areprovided with a number of outlet holes 74 communicating via a numberradial passages 75 with an annulus 76 between the turning body and thevessel hull. In this embodiment, however, a number of passages 41 arearranged in the vessel hull, in a manner corresponding to theembodiments of FIGS. 2 and 9, these passages extending between theannulus 76 and a water intake in the vessel. The water intake may beconstituted by a wing tank 42 in a manner corresponding to that of FIG.2, wherein a pump 44 which is coupled to a pipeline 45, is placed at thebottom of the wing tank. A corresponding water intake or a wing tank maybe arranged in the vessel on the opposite side of the well 90 inrelation to what is shown in FIG. 13.

[0068]FIG. 14 shows a horizontal section along the line XIV-XIV in FIG.13, and shows a possible arrangement of production risers 28, seawaterrisers 30 and anchor line fastening points in the turning body 70. Asshown, six production risers 28, six seawater risers 30 and twelve guidetubes 94 for anchor lines are arranged along respective concentriccircles. Each of the seawater risers 30 outside of the outlet holes 74is surrounded by a passage or an annulus 95 communicating with theannulus 76 via an associated passage 75.

[0069]FIG. 15 shows a sectional view corresponding to that of FIG. 14,but of an alternative embodiment with respect to the connection betweenthe riser outlets 74 and the passages 75. Instead of individual passagesor annuluses 95 around the risers 30, there is arranged a common annulus96, so that the outlet openings 74 of the risers debouch directly intothis annulus.

[0070] In operation of the system according to the invention, as thewater flows from the inlet of the seawater risers to the surface, thereis generated a difference in pressure from the inside to the outside ofthe risers. This difference in pressure is caused by the friction lossesand will increase from zero at the inlet to approximately the differencein pressure caused by the difference in static head between the insideand the outside of the water intake/wing tank at the buoy or turretposition.

[0071] The external pressure will tend to collapse the risers, and therisers will have to be designed with a sufficient thickness or with asuitable reinforcement to prevent the risers from collapsing.

[0072] The risers will also be subjected to movements caused by themovements of the vessel. Other forces are induced by wind, waves andforces caused by water currents. Due to the large diameter of the pipesand the induced movements and forces, the risers will be expensive tomanufacture. It may therefore be more economic or more technicallyfeasible to install the pumps at a sub-sea pumping station.

[0073] The pumps may be installed at the seabed or thereabove, dependingon the water depth and the optimum shape of the riser system. When thepumps are installed inside the risers or supply water into the risers ata certain depth, the internal pressure in the risers will be higher thanthe external water pressure above the location of the pump unit. As theriser no longer needs to be dimensioned to prevent collapse caused bythe external overpressure, it can be made as a less expensive “soft”pipe. A “soft” pipe will also be less stressed by vessel movements thana rigid pipe.

1. A system for supplying cooling water to a process on board a floatingvessel for the production of hydrocarbons, wherein the vessel (1) isanchored by means of a bottom-anchored turning unit (20) mounted in areceiving space (7) in the hull (34) of the vessel and allowing turningof the vessel (1) about the turning unit, and wherein the turning unit(20) supports a swivel unit (24) for the transfer of hydrocarbons fromproduction risers (28) extending between the seabed and the turning unit(20), the system comprising a conduit means (30) depending from thevessel (1) to a depth for taking in cooled sea water, and a pump means(44) for pumping of the sea water from the conduit to a place of use forthe process, characterized in that the turning unit (20) is designed asa seawater swivel, the unit being provided with one or more passages(29) for receiving upper end portions of respective seawater risers (30)constituting the conduit means, and with a means for transferring seawater from the upper end portions of the risers (30) to an annulus (31)arranged at the boundary surface between mutually movable parts (21, 22)of the turning unit (20) or between the turning unit (20) and the vesselhull (34), and communicating with one or more passages (41) arranged inthe vessel hull and leading to said place of use, a seawater sealingmeans (37, 39) being arranged on each side of the annulus (31).
 2. Asystem according to claim 1, wherein the turning unit is a two-partunderwater buoy (20) comprising a bottom-anchored central member (21)and an outer buoyancy member (22) rotatably mounted on the centralmember, and wherein the receiving space (7) for the turning unit isarranged at the bottom of the vessel (1), characterized in that thepassages for the risers (30) are arranged in the central member (21) ofthe buoy, and that the means for transferring sea water from the upperend portions of the risers (30) comprises said annulus (31) arrangedbetween the central member (21) and the outer buoyancy member (22) ofthe buoy (20), and a number of radial passages (32) arranged in thebuoyancy member between this annulus and an additional annulus (33) atthe boundary surface between the turning unit (20) and the vessel hull(34).
 3. A system according to claim 1, wherein the turning unit isconstituted by a turret (70) and the receiving space (90) for the turret(70) is arranged at the bottom of the vessel (1), characterized in thatthe means for transferring sea water from the upper end portions of therisers (30) comprises a number of radial passages (75) arranged in theturret (70) and communicating with said annulus (76) at the boundarysurface between the turret (70) and the vessel hull.
 4. A systemaccording to one of the claims 1-3, characterized in that said passages(41) in the vessel hull debouch into a water intake (42) constituted bya wing tank on each side of the vessel (1).
 5. A system according toclaim 4, characterized in that said pump means comprises one or morepumps (44) arranged at the bottom of each of the wing tanks (42) andwhich are connected to a respective conduit (45) for supplying water inthe wing tank (42) to said place of use.
 6. A system according to claim5, characterized in that each of the pumps (44) is arranged in a suctionextension well (55) arranged at a level below the bottom of the wingtanks (42).
 7. A system according to one of the claims 1-3,characterized in that the pump means comprises a number of pumps (56)arranged in respective ones of the sea water risers (30) at a locationbelow the turning unit (20).
 8. A system according to one of the claims1-3, characterized in that the pump means comprises a number of pumps(61) arranged in a dry space (60) above the turning unit (20), each ofthe pumps (61) being connected to a respective one of said passages (64)in the vessel hull (34) and to a respective conduit (59) for supplyingsea water from the risers (30) to said place of use.
 9. A systemaccording to claim 1, wherein the turning unit is constituted by aturret (70) and the receiving space (71) is arranged at a level abovethe water surface (72), characterized in that the pump means comprises anumber of pumps (80) arranged in respective ones of the seawater risers(30) at a chosen level below the water surface (72).
 10. A systemaccording to one of the preceding claims, characterized in that theseawater risers (9) extend between the turning unit (6) and the seabed(3) and with their lower end are connected to an anchoring means (10) onthe seabed.
 11. A system according to claim 9, characterized in that theseawater risers (9) as well as the production risers (8) comprise anupper flexible part which, at its lower end, is connected to a submergedbuoyancy unit (11), and a lower part extending between the buoyancy unit(11) and the seabed (3).
 12. A system according to claim 8 or 9,characterized in that the pump means is constituted by a pump stationlocated on the seabed.